(1) Field of the Invention
The present invention relates generally to a method of forming a semiconductor device, and more particularly to form an MOS transistor with reduced junction depth using a combination of multiple-pulsed laser anneal and rapid thermal anneal.
(2) Description of the Prior Art
Ion implantation process is essential to fabricate modern integrated circuits. Doping or otherwise modifying silicon or other semiconductor wafers relies on this technology, which involves generating the required beam of ions and implanting them into the substrate so that they come to rest just below the semiconductor surface. Most commonly implanted species are: antimony, arsenic, boron, boron difluoride, germanium, and phosphorous. Currently, ion implantation is used to form source and drain regions, shallow extension junctions between the channel and source/drain contacts, and electrically active poly-silicon gate electrodes. Ion implantation is always followed by an annealing step to heal the damage that occurs when ions occupy the interstitial spaces in the semiconductor crystal lattice during implanting.
As MOSFET devices are scaled down below 100 nm, highly doped ultra-shallow junctions are necessary for high current drive capability with acceptable short-channel performance. In order to minimize short-channel effect and maximize device performance at the same time, source/drain (S/D) engineering associated with diffusion profiles of source/drain junction are explored. Analysis shows that the critical parameter for reducing the resistance of the S/D extension junction is the dopant diffusion slope rather than its maximum doping level. In other words, development of advance process technology for achieving a box-shaped profile is a more efficient way to sustain lower junction resistance rather than pursuing higher doping level that may cause problems of junction depth control.
With the conventional junction formation by ion implant and rapid thermal anneal, it is very difficult to obtain highly steep ultra-shallow junction profiles because the interaction between implantation induced point defects and dopant atoms during annealing can considerably broaden the profile shape through the mechanism of transient enhanced diffusion (TED). Laser annealing with pre-amorphization implant (PAI) has received considerable attention as a potential solution to achieve low-resistance, ultra-shallow box-shaped SD extension junctions. However, the method has integration issues related to gate shape distortion due to inherently inhomogeneous annealing process that takes place within a few hundred nanoseconds. In the literature, pulse laser annealing in combination with PAI has been discussed with limited success. Unfortunately, the method has major integration problems because the high-energy fluence used for sufficient dopant activation and defect removal gives rise to melting of the poly-silicon gate. Although low energy implants are needed to reduce junction depths, the lowering of energy implies a trade-off between shallower junctions and higher junction resistivity.
U.S. Pat. No. 5,399,506 describes a process wherein shallow junction with reduced junction leakage is achieved by the combination of ion implantation and low temperature annealing (600° C. for 1 hour) to reduce point defects and pulsed laser (700 mJ/cm2, 44 nsec pulse width) irradiation to activate the implanted ions.
U.S. Pat. No. 5,937,325 describes a process to form low resistance titanium silicide gates by using a laser anneal process in two steps: first laser anneal converts the deposited titanium layer to a high resistivity titanium silicide layer and the second laser anneal (after removing the un-reacted titanium layer) converts the high resistivity silicide layer into low resistivity titanium silicide phase.
U.S. Pat. No. 6,100,171 describes a laser annealing process for removing fluorine from a gate conductor and thereby reduce boron penetration. During anneal, laser energy is such as to melt a portion of the gate conductor facilitating the removal of the fluorine that is incorporated in the gate during BF+2 implantation. In another embodiment, rapid thermal anneal (RTA) follows laser anneal process for activating the dopants in the S/D regions.
U.S. Pat. No. 6,365,476 B1 describes a laser thermal process wherein after the first implant and anneal to form S/D regions and after the removal of sidewall spacers around the gate structure, a blanket pre-amorphization implant is performed to form S/D amorphized extension regions. After depositing a layer that is opaque to a select laser wavelength, the substrate is then irradiated to selectively melt the amorphized S/D extensions.